This invention relates to the stereoselective epoxidation of cyclic 4-hydroxy olefins and, more specifically, to the stereoselective epoxidation of terpinen-4-ol.
Epoxidation is the conversion of olefins (or other substances containing carbon-carbon double bonds) into epoxy (oxirane) compounds. An epoxidation is considered stereoselective when its epoxide product comprises a substantial percentage of a single one of its possible isomeric products.
Base catalyzed epoxidation of olefins has been described previously by Payne in U.S. Pat. No. 3,053,856, in which organic nitrile compounds and hydrogen peroxide were employed to generate the active epoxidizing agent peroxycarboximidic acid in situ. However, only moderate product yields were obtained. Solvent concentrations of greater than 50% by weight were necessary, and concentrations greater than 70% were preferred. This necessity for high solvent concentrations gave rise to poor reactor efficiencies. Subsequent to publication of the patent, Carlson (J. Org. Chem., 36, 3832-33, 1971) taught that a peroxycarboximidic acid generated in situ by methods such as that disclosed by Payne is a less selective and more reactive epoxidizing agent than a chemically-similar peroxycarboxylic acid fully synthesized apart from the epoxidation reaction sequence then added thereto. When peracetic acid is allowed to react with terpinen-4-ol an approximate three to one mixture of the cis: trans epoxies is obtained which is undesirable because of the large amounts of the trans isomer present in the final product.
More recently, Achmatowicz and Szechner (Carbohydrate Res., 50, 23-33, 1976) described a stereoselective epoxidation of unsaturated carbohydrates using a peroxycarboximidic acid. Again, yields were moderate. Excellent stereoselectivity was achieved only when three oxygen substituents on the same face of the molecule could hydrogen bond to the peroxycarboximidic acid, thereby preferentially directing the acid's attack on the carbon-carbon double bond from that face of the molecule. For isomeric carbohydrates having one stereochemically inverted center bearing an oxygen-containing substituent, a much lower stereoselectivity of epoxidation was observed. In addition, high solvent concentrations were required.
Gagnieu and Grouiller (J. Chem. Soc. Perkin Trans. I, 1009-11, 1982) reported similar results with other unsaturated carbohydrates. Only where three oxygen substituents were on the same side of the molecule could high stereoselective epoxidation be accomplished. Systems with two oxygen substituents gave much lower epoxidation stereoselectivity. Again, highly solvent-diluted systems were necessary to perform the reaction.
In European Patent Application No. 151,941 McElligott described the phase transfer epoxidation of terpinen-4-ol to obtain the corresponding cis epoxide product in yields of about 75%; the epoxidizing agents employed were peroxo metal catalysts used in conjunction with stoichiometric amounts of oxidant (H.sub.2 O.sub.2) under phase transfer conditions. A compromise between conversion and selectivity, highest selectivity being at lower conversions, and vice versa was shown.
Therefore, it is an object of this invention to stereoselectively epoxidize cyclic 4-hydroxy olefins. It is a further object of this invention to stereoselectively epoxidize terpinen-4-ol to cis-1,2-epoxyterpinen-4-ol. It is another object of this invention to effect such epoxidation without the employment of phase transfer reagents. It is still another object of this invention to effect such epoxidation with peroxycarboximidic acid. It is yet another object of this invention to effect such epoxidation with peroxycarboximidic acid which is generated in situ. It is a particular object of this invention to do so in a more commercially practical manner than possible employing previously-known techniques.